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First Look at the Mantle Beneath an Active Back-Arc Basin

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First Look at the Mantle Beneath an Active Back-Arc Basin Contrib Mineral Petrol (2002) 143: 1–18 DOI 10.1007/s00410-001-0329-2 Yasuhiko Ohara Æ Robert J. Stern Æ Teruaki Ishii Hisayoshi Yurimoto Æ Toshitsugu Yamazaki Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin Received: 27 April 2001 / Accepted: 10 October 2001 / Published online: 11 December 2001 Ó Springer-Verlag 2001 Abstract Two dives of the DSV Shinkai 6500 in the is no evidence of pervasive post-melting metasomatism Mariana Trough back-arc basin in the western Pacific or melt–mantle interaction. Spinel compositions plot in sampled back-arc basin mantle exposures. Reports of the field for abyssal peridotites. Clinopyroxenes show peridotite exposures in back-arc basin setting are very depletions in Ti, Zr, and REE that are intermediate limited and the lack of samples has hindered our between those documented for peridotites from the understanding of this important aspect of lithospheric Vulcan and Bouvet fracture zones (the American– evolution. The Mariana Trough is a slow-spreading Antarctic and Southwest Indian ridges, respectively). ridge, and ultramafic exposures with associated gabbro The open-system melting model indicates that the Ma- dykes or sills are located within a segment boundary. riana Trough peridotite compositions roughly corre- Petrological data suggest that the Mariana Trough spond to theoretical residual compositions after 7% peridotites are moderately depleted residues after partial near-fractional melting of a depleted MORB-type upper melting of the upper mantle. Although some peridotite mantle with only little melt or fluid/mantle interactions. samples are affected by small-scale metasomatism, there The low degree of melting is consistent with alow magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma Y. Ohara (&) supply rate at spreading segments. Ocean Research Laboratory, Hydrographic Department of Japan, Tokyo 104-0045, Japan E-mail: [email protected] Tel.: +81-3-35414387 Fax: +81-3-35413870 Introduction R.J. Stern Department of Geosciences, The Philippine Sea occupies a large part of the Western University of Texas at Dallas, Pacific, and is composed of several large back-arc basins. Richardson, TX 75083-0688, USA The Pacific Plate subducts beneath the Philippine Sea T. Ishii Plate along the Izu–Ogasawara–Mariana Trench Ocean Research Institute, (Fig. 1a). This region evolved through several episodes University of Tokyo, of arc formation, rifting and back-arc spreading (Karig Tokyo 164-8639, Japan 1971). The Kyushu-Palau Ridge and West Mariana H. Yurimoto Ridge are remnant arcs separated by the Parece Vela Department of Earth Sciences, Basin. The active arc is separated from the West Mari- Tokyo Institute of Technology, Tokyo 152-8551, Japan ana Ridge by an active back-arc basin, the Mariana Trough (Fig. 1a, b). The Mariana Trough is a slow- T. Yamazaki Institute for Marine Resources and Environment, spreading back-arc basin (Fryer 1995) that is propa- National Institute of Advanced Industrial Science gating northward through the Mariana arc system and Technology, Tsukuba 305-8567, Japan (Martinez et al. 1995). This is a back-arc basin where the Present address: Y. Ohara composition and evolution of basaltic lavas have been Department of Geology and Geophysics, studied extensively (Hart et al. 1972; Hawkins and Woods Hole Oceanographic Institution, Melchior 1985; Sinton and Fryer 1987; Volpe et al. 1987, Woods Hole, MA 02543-1539, USA 1990; Hawkins et al. 1990; Stern et al. 1990; Stolper and Editorial responsibility: T.L. Grove Newman 1994; Gribble et al. 1996, 1998). 2 Fig. 1. a Satellite altimetry map showing the tectonic feature of the Philippine Sea(datafrom Sandwell and Smith 1997). The rectangle shows the location of b. b Shaded structural image of the Mariana arc system illumi- nated from 90° (data from the predicted bathymetry of Smith and Sandwell 1997). Heavy dashed line indicates NVTZ (northern volcano tectonic zone) and SVTZ (southern vol- cano tectonic zone), and heavy plain line indicates spreading section of the Mariana Trough. Amagmatic rifting occurs in the Central Graben. The rectangle shows the location of Fig. 2 Reports of peridotite exposures in back-arc basins are limited to the Mariana Trough (Bloomer et al. Geologic background and sample location 1995; Stern et al. 1996, 1997) and the Parece Vela Basin (Ohara et al. 1996). Of these, only the Mariana Styles of extension vary along the strike in the Mariana Trough is presently active. One dredge haul during the Trough, which can be divided into four parts (Fig. 1b; 1991 Tunes 7 expedition aboard the R/V Thomas Martinez et al. 1995; Gribble et al. 1998): Washington first recovered back-arc basin mantle 1. The northern volcano-tectonic zone (NVTZ) – char- rocks (Stern et al. 1996). Two dives of the DSV acterized by asymmetric rifting accompanied by fis- Shinkai 6500 in 1996 mapped and sampled exposures sure eruption along normal faults and point source of back-arc basin mantle in the Mariana Trough volcanism. (Stern et al. 1997). Stern et al. (1996) argued by 2. The southern volcano-tectonic zone (SVTZ) – char- analogy with mid-ocean ridges (Mutter and Karson acterized by localization of volcanic and tectonic 1992; Tucholke et al. 1998) that slow back-arc exten- activity and southward-progressive separation of the sion may be accomplished amagmatically. Although extension axis from the arc. amagmatic tectonics may be common in certain phases 3. The Central Graben – deeps produced by mechanical of back-arc basin evolution based on the recent extension that expose back-arc basin lower crust and mapping study of the Philippine Sea (Ohara et al. upper mantle (Stern et al. 1996). 2001), little is known at present about the importance 4. South of 19.7°N – extension occurs by slow seafloor of this process. spreading. The basin is widest at 18°N, where full- In this paper, we describe in detail, for the first time, spreading rates are between 3 and 4.4 cm/year the petrology of upper mantle peridotite from the (Hussong and Uyeda 1981), corresponding to those Mariana Trough, and use petrographic, mineral chemi- of slow-spreading mid-oceanic ridges. cal, and ion microprobe trace element compositions to understand how these formed. In addition to the appli- These variations mark different stages in the evolu- cation of this information in understanding back-arc tion of the back-arc basin because of northward-pro- basin lithosphere evolution, these data fill a critical hole pagating rifting and spreading. Martinez et al. (1995) in our understanding of ophiolites. The lack of samples argued that true seafloor spreading exists only south of of back-arc basin mantle has limited our understanding 19.7°N, although Yamazaki et al. (1993) argued that of ophiolite formation because back-arc basins are spreading occurs as far north as 22°N based on magnetic thought to be very common in the inventory of data. Rifting occurs in the NVTZ, the SVTZ, and the ophiolites (Bloomer et al. 1995). Central Graben (Martinez et al. 1995; Baker et al. 1996). 3 Mariana Trough (5,700 m), and is remarkably deep even compared with the anomalously deep 3.6-km mean depth of the Mariana Trough back-arc basin spreading axis (Stern et al. 1996). A single dredge in the Southern Basin was performed during the 1991 Tunes 7 expedition aboard the R/V Thomas Washington. The dredge (Fig. 2; D45; 20°02.5¢N, 144°04.0¢E; 5,175–4,800 m depth) recovered a diverse suite of upper mantle and crustal rocks, includ- ing peridotites, gabbros, and felsic plutonic rocks. The petrologic data for the Central Graben crustal suite is clearly more similar to MORB or back-arc basin crust than it is to arc crust (Stern et al. 1996). The isotopic data for D45 crustal samples are also indistinguishable from basalts from portions of the Mariana Trough undergoing seafloor spreading and distinct from those of the Mariana arc. Stern et al. (1996) concluded that the dredge recovered samples from a talus pile at the base of the eastern scarp of the Central Graben, and argued that the Southern Basin exposes a back-arc basin crustal and upper mantle section, with the crustal suite remarkably heterogeneous with gabbros, basalts, and felsic plutonic Fig. 2. Bathymetry of the Southern Graben and track lines of DSV rocks. Shinkai 6500 dives 358 and 359. D45 represents the approximate The DSV Shinkai 6500 dives 358 and 359 were line of the Tunes 7 dredge 45 by R/V Thomas Washington (Stern conducted in 1996 in the Southern Basin in order to et al. 1996). Contours in 100-m intervals understand this unique upper mantle section (Fig. 2). Dive 358 recovered 14 samples from five sites, in- In the Central Graben, individual grabens are located cluding gabbros, basalts, tonalite, and serpentinites. within a20–30-km-wide sinuous bathymetric low This dive is inferred to have sampled debris flows that between 21° and 19.7°N (Fig. 1b; Stern et al. 1996). The originated from higher up the scarp (Fig. 3; Stern et al. Central Graben is well delineated in the free-air gravity 1997). In contrast, dive 359 traversed mountainous anomalies as a band of relative lows between about 0 outcrops and steep exposures. Twenty-four samples and 25 mgal. Free-air gravity anomalies from distinct recovered from six sites along the dive 359 track sub-lows within this band and reflect the bathymetric represent an upper mantle suite of 14 peridotites and sub-basins (Martinez et al. 1995). Individual grabens are 10 gabbros, inferred as younger dikes or sills (Table 1; separated by along-strike bathymetric highs. Seismic Fig.
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